The present invention is in the general field of multiple photoflash lamp arrays, such as flash cubes and planar arrays, which employ an operatively associated electrical circuit to enable the flash lamps to be fired individually and in sequence. Photoflash units of this type are well known and described in the U.S. Pat. Nos. 3,990,832 and 3,990,833, both assigned to the assignee of the present invention, and the flash sequencing electrical circuit means therein disclosed employ solid state radiant energy switch devices to enable the desired switching action. In accordance with said prior art photoflash lamp units, there is included a plurality of flash lamps, a circuit board with an electrical circuit into which said lamps are arranged to fire individually and in sequence, and a plurality of solid state radiant energy switching devices as part of the electrical circuit. Each device is located adjacent one of the lamps to receive radiant energy emitted by that lamp, the device being a mass of a composition preferably comprising silver oxide, a carbon-containing silver salt and a humidity resistant organic polymer binder. This switch initially has a high electrical resistance ("off" condition) and after undergoing thermochemical change as a result of the radiant energy emitted by the lamp, the silver oxide and silver salt are converted to metallic silver which has a relatively low electrical resistance ("on" condition). As further previously known, silver oxide if used alone as a silver source renders the switch overly sensitive in reacting too vigorously when converting to the low electrical resistance state. By substituting a carbon containing silver salt as part of the silver source, the switch becomes less sensitive when converting yet the switch is still sufficiently sensitive to convert reliably when the adjacent lamp is flashed. If the silver source consists of a carbon containing silver salt alone, however, the switch may not be sufficiently sensitive to convert reliably when exposed to thermal energy of the adjacent flash lamp. The prior art switch material also comprises an admixture of the aforementioned constituents having various portions of binder to silver source and with said proportions being generally dependent upon chemical composition of said constituents. In one preferred embodiment, the switch material composition is formulated to provide the desired sensitivity of operation with a known flash lamp construction by utilizing stoichiometric proportions for the binder and silver source constituents. More particularly, the desired conversion of silver oxide (Ag.sub.2 O) and/or a carbon containing silver salt such as silver carbonate (Ag.sub.2 O) to elemental silver is said to take place with accompanying combustion of the organic binder wherein at least part of the oxygen required from said combustion is obtained from chemical reduction of the silver source material. A complete reduction of the silver source material in said manner is thereby promoted if stoichiometric principles are applied for the proportions of silver source material and binder in a mixture. As an example of these principles, it can be seen that each Ag.sub.2 CO.sub.3 or Ag.sub.2 O molecules furnishes one oxygen atom for combustion of the organic binder thereby requiring one or two oxygen atoms for each carbon atom (depending upon whether CO or CO.sub.2 is a reaction product) as well as one-half an oxygen atom for each hydrogen atom in the binder. By following said above defined stoichiometric ratios, the weight fractions of the specified constituents are accordingly adjusted to provide the desired complete conversion of the silver source to elemental silver.
Unfortunately, it has been found that such stoichiometric formulation of the switch material composition still leads to such an overly vigorous reaction that switches are blown off the circuit board. This loss causes an interruption in the electrical circuit path precluding further actuation of unflashed lamps in the present circuit arrangement. A method employed to avoid such circuit impairment has been to increase the thickness of the switching devices deposited on the circuit board so that some converted switch material will still reamin, but such method understandably increases manufacturing costs. It is desirable, therefore, to provide a more reliable switch operation in this regard without necessitating a significant substitution of material compositions or requiring a substantial departure from methods now employed to form the switch device.